We learned that spiders have eight eyes. Do they
see through all 8 eyes or do they only use two
like us or do they see completely different from us?

Question Date: 2001-03-15

Answer 1:

Great question. You're right, most spiders do have
eight eyes - but not all of them! Some spiders
have two eyes while others have four or six. There
are over 34,000 species of spiders and one of the
ways we separate them out into different families
is by the number of eyes they have! With a few
exceptions (see below), if spiders have more than
two eyes, they can see through all of them. And,
roughly speaking, they have eyes very similar to
us - called "simple" eyes. However, what they see
through each of the eyes can be very different.
Generally speaking, the eyes centered on the head
and close to the spider's face are used to detect
the size, shape, and color of an object. Eyes
further away from the center of the face (along
the side of the head) are used to detect motion.
So, if an object moves, say, a cricket that could
be food, the spider is usually going to detect it
with it's peripheral (side) eyes. Subsequently,
the spider will learn more about the nature of the
object (size, color, etc.) once it is facing it.
Here's another interesting fact: six years ago
scientists in Eastern Europe uncovered an
underground cave that had been sealed for
thousands of years. Inside they found spiders with
no eyes at all! Upon studying them more closely,
they discovered that these spiders had eyes when
they were first born, but over time the eyes went
away (atrophied)!! They hypothesized that these
spiders were descendant from spider species that
used eyesight (as do most spiders) but after
thousands of generations of living in the dark,
they evolved such that their eyes were no longer
used in adulthood!

Amazing, huh?

Answer 2:

I can tell you a little bit about spider
eyes.Maybe you can find out the rest by going to
the library. Then you can teach me something
about spiders. Scientists think that spiders
see completely different than we see things. We
humans are so used to seeing with two eyes that it
is hard to imagine what it must be like to see
with more than two. So, scientists try to
understand how spiders use their eyes by doing
experiment where they change the amount of light,
the direction of light, the color or colors of
light, shadows, pictures, etc. By doing these
experiments, they have out some very interesting
information.First, the number of eyes a spider
has can vary from none up to eight. Some spiders
that live in caves where there is no light have no
eyes. For most spiders, six and eight eyes is
common. Second, most spider eyes are used to
see movement. To get an idea of what that is
like, face a window with birght light shining
through it, and hold up a piece of paper in front
of your eyes so it covers your eyes. Have a
friend walk between you and the window. Do you
see how you can tell something is moving but you
can't tell exactly what it is? That's how most
spiders see. Some spiders that hunt other animals
can probably see color and shapes, so they can
tell what they are looking at. Remember, sight
is not the only way to tell what is going on in
the world. Try going to the library and looking
up spider in the encyclopedia, or looking up a
book on spiders. You'll find out that spiders
have other senses that are really good. Can you
tell me what they are?

Answer 3:

What a great question! This is a question that we
discuss in college-level biology courses, so
you're way ahead of the game by thinking of this
now, 10 years before you're in
college!

If something on an animal is
described as an eye that means that that thing can
at the very least "see" light. That is, the eye
may only be able to "see" when the animal is in
the light or not, but not actually "see" any
object. You can find out what it would be like to
have eyes like this by shutting your eyes and then
have someone turn the room lights on and off. Even
though your eyes are shut, you can still tell when
the room lights are on or off because some light
gets through your eyelids and so your eyes can
still "see" whether there is light around you or
not. However you are not really "seeing" anything.
Many animals have very simple eyes that function
like this, as light/dark detectors, and that is
all they need to survive. Isn't that
amazing!

However, spiders are a bit
more sophisticated than that. At least two of the
spiders eight eyes can "see" objects, sort of like
our eyes can, but not as well as our eyes can. The
other 6 eyes sometimes are only light/dark
detectors (but it depends on the type of spider).
The spiders with the best eyes of all are the
jumping spiders. They are common around Santa
Barbara. They are small and furry and sometimes
have a bright orange spot on their back (they also
jump). They can watch and follow non-moving bugs
that they want to eat. Most other spiders can only
"see" things that move. You can find out what this
type of vision is like by staring straight ahead
and not looking to the side while a friend of
yours moves an object about 2 feet away from the
side of your head. Keep looking straight ahead
while your friend holds the object about 2 feet
from your ear, so you can barely see it. Can you
describe what it is? Probably not. But, can you
tell when your friend moves the object or not?
Probably yes. The way we see things out of the
corner of our eyes is the type of vision most
lower-level animals have. If you catch a spider
you can test this out on the spider. See if it
seems to change its behavior in response to
non-moving things. Then see if it responds to you
waiving your hand around in one spot. Good luck,
and I wouldn't recommend doing this with a black
widow spider!

Answer 4:

I can see you're already thinking creatively about
this. You're right that even though both spiders
and humans have eyes, they don't necessarily "see"
in the same way. This site contains
information on spider eyes: spiders

Basically, the different eyes can give the
spider different information. In order to tell
how far away something is, you have to have at
least 2 eyes that are close together. This is
useful if you want to jump on your prey. You don't
want to jump too far or too close and miss it. Of
course, you don't want another animal catching
you, so it helps to have some eyes on the side so
you can tell if something is sneaking up on you.
You don't need to see what they look like, a
simple sensor that tells you something is moving
or blocking the light will do. This will also
tell you if there is a small, moving object off to
the side, like another prey animal.

None
of these eyes give a spider exactly the kind of
information you get from your eyes. The spider
probably does not see the detailed picture that
you do. If you wanted to find out what a spider
can see, how would you test this? Here's an
easier question, if you wanted to tell whether a
dog could tell red from blue, how could you tell?
(Hint: think about training it to bark everytime
it saw a certain thing.)